h264dec: be more explicit in handling container cropping

h264dec: be more explicit in handling container cropping

The current condition can trigger in cases where it shouldn't, with
unexpected results.
Make sure that:
- container cropping is really based on the original dimensions from the
  caller
- those dimenions are discarded on size change

The code is still quite hacky and eventually should be deprecated and
removed, with the decision about which cropping is used delegated to the
caller.